According to your evalbum page www.evalbum.com/4001, you have 50 CALB 130ah LiFePO4 cells. With 50 cells, 120v is 2.4vpc on average. I'm not a lithium expert, but that sounds pretty low to me.
In fact the CALB datasheet I found online said 2.5v was the low limit for these cells. It also recommends a 0.3C nominal current, which is only 39a, with a max of 1000a for less than 10 sec with no duty cycle specified. The datasheet has discharge voltage curves for up to 1C (130a) suggesting, but not stating, that the manufacturer might consider that an acceptable current. That chart doesn't show the voltage falling to 2.6v until the cell has delivered a little over 140ah at that current, but it also doesn't say anything about temperature. All that said, let's assume that the prudent EVer will try not to go much above the recommended nominal current of 39a. However, your evalbum entry says that on the highway at 60+mph you're seeing current in the 200a range. You also say you're using 500Wh/mi. Taking an average of 45mph and solving for motoring current, we get 173a. That's still pretty high. Again, I defer to the experts on this, but my first thought is that your prolonged use at relatively high currents may have accelerated the depreciation of your battery. You may have chosen too small a battery for the vehicle. The solution is either larger cells, or more of the current size. You also mention on your evalbum page that when you charge, the charger only puts 117ah into the battery. The way it's worded suggests that previous charges put more than that in. For good battery cycle life, you shouldn't be discharging past 80% DOD (104ah). However, amp-hours charged may not indicate actual DOD. Do you have a measurement of your actual consumption? If you're routinely discharging below 80%, that could also be a factor in "battricide." To carry out diagnostics, you don't need a dynamometer, but rather a dummy load connected in place of the motor (or possibly motor and controller). You want something that draws the current you need for testing at your battery's voltage without having the EV move - in this case, between your nominal road current (173a?) and your peak road current of 300a. One "classic" EV dummy load is an electric water heater element suspended in a barrel of water. Others who've done it may be able to give you more details. My dummy load uses resistance heating elements from derelict heat pumps, split in half with the halves wired in parallel for lower voltage, and cooled with a fan. It has its own contactor. (If you connect your dummy load ahead of the controller, or if you have AC drive, you definitely need a contactor.) Charge the battery. Apply the load and measure the voltage at each cell. Also check across connections in the hope that maybe one is poor (though you'd probably be seeing symptoms of severe heating if that were happening). Continue to monitor individual cell voltages as the battery discharges. This test should show you whether your problem is a few "stinker" cells, or the entire battery. David Roden EVDL Administrator http://www.evdl.org/ _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
